It doesn’t happen often in nature, but now and then, a wild Atlantic salmon (yes, there are still a few left) mates with a brown trout and has hybrid offspring.

This ability to reproduce between species had some Canadian scientists curious: If a genetically modified Atlantic salmon were to come in contact with a brown trout, would it too be able to have little transgenes babies? The answer is yes, according to a new study published in the Proceedings of the Royal Society B. And it turns out that those offspring carry the genetically inserted trait that allows them to grow faster than their Mother-Nature-made cousins. Much faster. In fact, the hybrid offspring outgrew their genetically tweaked parents as well.

“When the fish were placed in a mocked-up stream inside the laboratory, the researchers found that the hybrids were out-competing both the genetically modified salmon and wild salmon, significantly stunting their growth,” writes Rebecca Morelle, science reporter for BBC News.

The fact that these fast-growing fry even exist and can thrive may contradict the FDA’s own environmental assessment report, which says genetically engineered salmon’s ability to survive and reproduce in the wild is extremely remote, calling into question the agency’s finding of no significant impact.

Ron Stotish, AquaBounty’s CEO, responded to the study in a statement, reminding the public that the AquaAdvantage salmon are all female, triploid (which means they are sterile), and will be raised on land-based systems.

But the researcher’s paper concludes that, “hybridization of transgenic fishes with closely related species represents potential ecological risks for wild populations and a possible route for introgression of a transgene, however low the likelihood, into a new species in nature.”

“There’s been a whole debate about whether there would be any potential ecological consequences,” says Leonard. “This is a strong smoking gun that there could be. Hybrids occurred. They survived, and what’s more troubling, is that they were more competitive than the wild or pure GE fish.”

“My question is what does the FDA say now? This paper was not part of the record. Does the FDA conclude that there’s still nothing to worry about?” he asks.

FDA spokesperson Siobhan DeLancey says that a decision on the application by AquaBounty for its genetically engineered salmon still has not been made.

“We’re considering all the available science, including recently published papers such as this one,” she says.

While the new study and FDA’s consideration of it may potentially disrupt or delay the GMO salmon’s approval, consumer advocacy group Campaign for Genetically Engineered (GE)-Free Seafood continue with plans trying to keep the fish out of supermarkets. Yesterday the group announced that they’ve now secured a total of 59 retailers who pledge not to sell genetically engineered seafood, adding Target, Giant Eagle, H-E-B, and Meijer to the growing list.

“When it looked like the FDA was going to approve the fish, we took our fight to the public to make sure there wouldn’t be a market for the genetically engineered fish if it’s approved, or at the very least, that customers could know where they could go to buy GE-free seafood if they want,” said Erick Hoffman, food and technology policy campaigner for Friends of the Earth.

Next on their list? Chefs, restaurants and seafood companies.

“This is a long-term effort for us,” says Hoffman.

Indeed, but given Ron Stotish’s nearly two decade-long effort to get a genetically modified salmon onto your dinner plate, we’re wondering just how much longer the fight will be focused on this fish, knowing dozens more are in the works around the globe.